An example of conventional switching devices for diaphragm pumps is described in Japanese Utility Model Publication No. 35748/1986 which calls for providing a detent mechanism at the end of a spool used for switching the operations of a diaphragm pump in order to prevent the spool from standing still at the neutral point.
Such a conventional detent mechanism calls for convexes around the shaft at the end of the spool and propelling balls by spring pressure to the surface of the shaft at the end of the spool by means of compressed coil springs. And when the spool moves in an axial direction, the spring pressure in the direction of the radius of each ball works upon the slope of each of said convex, pushing the spool shaft towards one end of the shaft with the neutral point at the center so that the spool does not stop at the neutral point.
As the detent mechanism of a conventional device comprises convexes and compresses coils springs, when the pressure of the compressed coil springs is strong, frictional resistance between the convexes and the balls becomes strong, causing abrasion. As a result, problems lowering durability are evident. When such abrasion appears, there will be further problems in that the spool, which is fitted inside the sleeve so as to be able to rotate, becomes unable to rotate so that the abrasion continues at the convexes over only a limited portion of the balls.
On the other hand, if the pushing pressure of the coil springs is weak, the force from the balls upon the convexes becomes insufficient and thus, the switching operation of the spool becomes unreliable.
The objective of the present invention is to ensure both prevention of the detent mechanism's abrasion and the thorough switching operation of the spool.